Dissertations / Theses on the topic 'Al-Mg-Si-Cu alloys'

This thesis systematically investigates the effects of a recently developed modified ageing procedure of aluminium alloys, termed the T6I6 temper, on the microstructural development and mechanical properties of the Al ??? Mg ??? Si - Cu alloy 6061. For the T6I6 temper, a conventional single stage T6 temper is interrupted by an ageing period at a reduced temperature (65??C) to facilitate secondary precipitation, before resuming the final ageing at the temperature of the initial T6 treatment. The T6I6 temper was found to cause simultaneous increases in tensile properties, hardness, and toughness as compared with 6061 T6. Al ??? Mg ??? Si ??? Cu alloys are medium strength alloys widely used in the automotive industry and their further improvement is underpinned by stringent demands for weight reduction placed on the transportation industry in recent years. The potential for further improvement of the mechanical properties was found in the control of secondary precipitation that may take place even in some fully aged alloys when exposed to reduced temperatures. The overall improvement in the mechanical properties of 6061 T6I6 was attributed to the formation of finer and more densely dispersed precipitates in the final microstructure. The refinement of precipitates was facilitated by control of the precipitation processes and gradual evolution of the microstructure throughout each stage of the T6I6 treatment. The results indicated that the concentration and the chemical environment of the vacancies controlled the precipitation processes in this alloy. Findings also show that the proportion of the different precipitate phases present in the final microstructure, as well as the amount of the solute in these precipitates, can be controlled and modified utilizing secondary precipitation. A number of analytical techniques were used in this study. The evolution of the microstructure was studied using Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Three Dimensional Atom Probe (3DAP). Vacancy-solute interactions were studied using Positron Annihilation Lifetime Spectroscopy (PALS) and 3DAP. The distribution of the solute was studied using 3DAP and Nuclear Magnetic Resonance (NMR). Differential Scanning Calorimetry (DSC) was used to identify precipitation reactions and to determine the stability of vacancy-associated aggregates.

Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter. Increasing the knowledge about the heat treatment process is one task in the direction towards this goal. The aim of this work is to investigate and model the heat treatment process for Al-Si casting alloys. Three alloys containing Mg and/or Cu were cast using the gradient solidification technique to achieve three different coarsenesses of the microstructure and a low amount of defects. Solution treatment was studied by measuring the concentration of Mg, Cu and Si in the α-Al matrix using wavelength dispersive spectroscopy (WDS) after various times at a solution treatment temperature. A diffusion based model was developed which estimates the time needed to obtain a high and homogenous concentration of alloying elements for different alloys, temperatures and coarsenesses of the microstructure. It was shown that the yield strength after artificial ageing is weakly dependent on the coarseness of the microstructure when the solution treatment time is adjusted to achieve complete dissolution and homogenisation. The shape and position of ageing curves (yield strength versus ageing time) was investigated empirically in this work and by studying the literature in order to differentiate the mechanisms involved. A diffusion based model for prediction of the yield strength after different ageing times was developed for Al-Si-Mg alloys. The model was validated using data available in the literature. For Al-Si-Cu-Mg alloys further studies regarding the mechanisms involved need to be performed. Changes in the microstructure during a heat treatment process influence the plastic deformation behaviour. The Hollomon equation describes the plastic deformation of alloys containing shearable precipitates well, while the Ludwigson equation is needed when a supersaturated solid solution is present. When non-coherent precipitates are present, none of the equations describe the plastic deformation well. The evolution of the storage rate and recovery rate of dislocations was studied and coupled to the evolution of the microstructure using the Kocks-Mecking strain hardening theory.

One of the serious problems encountered in the use of various materials in technology is the occurrence of fatigue phenomena as an undesirable material damage under cyclic mechanical load. For aluminium alloys this issue is of extremely high importance in case of their utilisation for aircraft purposes, e.g., where a very wide spectrum of frequencies occur. Moreover, the cyclic loading may be joined by the presence of specific electrolyte media. Therefore, the material candidates must be thoroughly examined in view of their sensitivity to fatigue as well as to corrosion fatigue. Usually, the Cu-containing alloy EN-AW 2024 T3 is applied besides 7075 T6 in Airbus aircrafts, but the weldable alloy 6013 T6 is considered to be a potential alternative. Referring to former investigations on the environmental sensitivity (ES) in the fatigue behaviour /1-6/ this paper brings up experimental findings as well as expanded considerations about damaging mechanisms and modelling. The situation with the alloy 6013 T6 is emphasized. The propagation of cracks under cyclic load in different environments, such as vacuum, air or aqueous media, is described by means of fracture mechanics. This enables discrimination in view of the influence of environmental factors and, hence, the participation of corrosion processes.

One of the serious problems encountered in the use of various materials in technology is the occurrence of fatigue phenomena as an undesirable material damage under cyclic mechanical load. For aluminium alloys this issue is of extremely high importance in case of their utilisation for aircraft purposes, e.g., where a very wide spectrum of frequencies occur. Moreover, the cyclic loading may be joined by the presence of specific electrolyte media. Therefore, the material candidates must be thoroughly examined in view of their sensitivity to fatigue as well as to corrosion fatigue. Usually, the Cu-containing alloy EN-AW 2024 T3 is applied besides 7075 T6 in Airbus aircrafts, but the weldable alloy 6013 T6 is considered to be a potential alternative. Referring to former investigations on the environmental sensitivity (ES) in the fatigue behaviour /1-6/ this paper brings up experimental findings as well as expanded considerations about damaging mechanisms and modelling. The situation with the alloy 6013 T6 is emphasized. The propagation of cracks under cyclic load in different environments, such as vacuum, air or aqueous media, is described by means of fracture mechanics. This enables discrimination in view of the influence of environmental factors and, hence, the participation of corrosion processes.

Orientador: Eugenio Jose Zoqui
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
Made available in DSpace on 2018-08-11T16:09:14Z (GMT). No. of bitstreams: 1 Benati_DaviMunhoz_M.pdf: 9368625 bytes, checksum: 4c58a987840759e499c7fd1c0db0d7f6 (MD5) Previous issue date: 2008
Resumo: Este trabalho teve co:mo objetivo avaliar a tixoconformabilidade de ligas Al-Xwt%Si2,5wt%Cu-0,5wt%Mg, abrangendo desde a obtenção das ligas tixofundidas até sua completa caracterização em termos de macro e microestrutura e comportamento viscoso. Ligas Al-1,Owt%Si-2,5wt%Cu-0,5wt%Mg, Al-2,Owt%Si-2,5wt%Cu-0,5wt%Mg, Al-4,Owt%Si2,5wt%Cu-0,5wt%Mg e Al-7,Owt%Si-2,5wt%Cu-0,5wt%Mg foram produzidas via ultra-refino de grãos utilizando o refinador Al-5,Owt%Ti-l,Owt%B. Os materiais foram submetidos a tratamentos térmicos de reaquecimento pelos.tempos de Os, 30s, 90s e 21Os, para duas condições de frações sólidas, 45% e 60%. As ligas contendo 1,Owt%Si, 2,Owt%Si e 4,Owt%Si mostraram-se. bastante próximas em termos de viscosidade aparente, no entanto, a liga com 4,Owt%Si apresentou melhor desempenho em termos de globularização, expressa pelo RQI. Já a liga contendo 7,Owt%Si mostrou-se dendrítica para todas as condições estudadas, o que dificulta sua aplicação nos processos de tixoconformação Os tempos de tratamento térmico agem no sentido de promover a globularização das partículas de fase primária, de forma que os melhores resultados de viscosidade foram alcançados para ligas submetidas aos tempos de 21 Os. As frações sólidas de 45% e 60% não apresentaram diferenças significativas em termos de RQI, porém as ligas contendo menor fração sólida apresentaram melhor desempenho em termos de viscosidade aparente, da ordem de 105Pa.s
Abstract: The goal of this work was to evaluate the thixoformability of Al-Xwt%Si-2.5wt%CuO.5wt%Mg alIoys, including the thixocasting of all alloys and their complete characterization in terms of macro and microstructures and rheological behavior. Al-l.0wt%Si-2.5wt%CuO.5wt%Mg, Al-2.0wt%Si-2.5wt%Cu-0.5wt%Mg, Al-4.0wt%Si-2.5wt%Cu-0.5wt%Mg and Al-7.0wt%Si-2.5wt%Cu-0.5wt%Mg alloys were produced by ultra-refining by addiction of Al5.0wt%Ti-1.0wt%B master alIoy. The materials were submitted to re-heating treatment for Os, 30s, 90s and 2l0s in two conditions, 45% al!d 60% of solid fraction. 1.0wt%Si, 2.0wt%Si and 4.0wt%Si alloys showed very similar results for apparent viscosity, however the 4.0wt%Si alloy showed better performance in terms of globularization, measured by RQI. Thixocast 7.0wt%Si alloy presented dendrictic structures for alI conditions analyzed, making its application too difficult for thixoforming processes. Re-heating treatment times promote the solid phase particles globularization, so that the alloys treated during 210s achieved better results for apparent viscosity. Both 45% and 60% solid fractions did not show significant differences in terms of RQI, but the alloys containing lower solid fraction showed better performance for apparent viscosity, in the order of 105Pa.s
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

The processing window is important for the semisolid processability of alloys. Applications of semi-solid metal (SSM) processing, especially aluminium alloys have been expanding for their excellent mechanical properties. However, the alloys well suited and commercially used for SSM processing today are limited in types. The main purpose of this Ph.D. project is to understand what makes an alloy suitable for SSM processing on both aspects of thermodynamics and kinetics. This research started with a fundamental study of binary alloys based on Al-Si, Al-Cu and Al-Mg systems (wt%): Al-1Si, Al-5Si, Al-12Si and Al-17Si; Al-1Cu, Al-2Cu and Al-5Cu; Al-0.5Mg, Al-3Mg and Al-5.5Mg. These are representative of Si, Cu and Mg contents in commercial alloys used for SSM processing. The Single-Pan Scanning Calorimeter (SPSC) and Differential Scanning Calorimeter (DSC) were used to investigate the liquid fraction changes during heating and cooling of these binary alloys. Thermo-Calc and DICTRA (DIffusion-Controlled TRAnsformations) software have been used to predict the fraction liquid versus temperature taking into account both thermodynamics and kinetics. Comparison of the predictions with experimental data revealed that the simulation results show the same pattern with experimental results in the fraction liquid-temperature relationship. However, the SPSC results are closer to the prediction than DSC curves are, even with the relatively large sample size associated with SPSC. This is potentially a significant result as predicting the liquid fraction versus temperature for the heating of a billet for semi-solid processing remains one of the challenges. The results also suggest that the fraction liquid sensitivity to time should be identified as a critical parameter of the process window for semi-solid processing in addition to the fraction liquid sensitivity to temperature. For microstructure investigation, microanalysis techniques, including Scanning Electron Microscopy (SEM) and micro-indentation testing, have been used on polished sections, and compared to theoretical predictions. In addition, some parts of this project are in cooperation with General Research Institute for Nonferrous Metals (GRINM), which aims to design and develop high performance semi-solid alloys. Thermodynamic analysis (both predictions and experiments) were carried out on thixoformed 319s (2.95Cu, 6.10Si, 0.37Mg, wt%) and 201 (4.80Cu, 0.7Ag, wt%) aluminium alloys. SEM techniques and Transmission Electron Microscopy (TEM) were used for the microstructural characterisation. The results showed that the DSC curves were sensitive to microsegregation in SSM alloys and resulted in a lower liquid fraction than the cast alloys calculated through the integration method from the DSC results. Al2Cu phase in SSM alloys 319s and 201 can be dissolved into matrix up to 0.4 % before melting temperature under 3K/min heating rate when compared with 10K/min heating rate. The DSC scan rate should be carefully selected as higher heating rate can inhibit dissolution of the intermetallic phases during heating leading to less accurate liquid fractions predictions.

Hardness measurements and Three-Dimensional Atom Probe (3DAP) were used to characterize the early stages of precipitation in three different Al-Mg-Si alloys (Al-0.50 wt%Mg-1.00 wt%Si) with different Cu contents (0.03 wt%, 0.15 wt%, or 0.80 wt% Cu). Heat treatments were chosen to simulate an industrial production line for car body-sheet material and included natural ageing (NA), pre-ageing at 80 °C (PA), paint-bake ageing at 180 °C (PB) and 10 second ageing at 180 °C (spike). The Cu content and the chosen heat treatments were found to influence the microstructural evolution of the alloy considerably. Based on the determined microstructures and matrix solute concentrations, mechanisms for the effect of NA, PA and Cu additions were proposed. NA had a deleterious effect on the PB hardening response, which was delayed dramatically after 20 minutes NA or longer. When the NA time was 1 minute, β" precipitates were formed within 30 minutes PB resulting in high hardness of the alloy. The delay with NA time was caused by a decrease in the nucleation rate of elongated precipitates during the subsequent PB. This decrease was thought to be due to a combination of a decrease in the matrix solute concentrations and clusters acting as vacancy sinks. PA before NA improved the PB response due to the formation of a high density of short elongated precipitates. Small Mg-Si clusters were detected after both NA and PA. Clusters formed during PA were found to be, on average, Mg-richer and larger than those formed during NA. Larger clusters were found to be more stable during PB and, upon PB, to grow into nucleation sites for elongated precipitates. Application of a spike before PA resulted in faster growth of clusters during PA. Growth of clusters and nucleation of short elongated precipitates during PB was found to be enhanced with increasing Cu content when no PA was given. Cu was found to be present in all precipitates and clusters in the alloy with the highest Cu content. These precipitates were thought to be precursors to the Q' phase.

Application prospects in the automotive industry have led to increasing studies on Al-Mg- Cu-Si alloys. In this thesis, nine Al-(1-3)Mg-(0-0.4)Cu-0.15Si-0.25Mn (in wt%) alloys with potential applications in packaging and automotive industries have been investigated. By means of mechanical testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), several mechanisms was identified that influence the final strength of cold rolled alloys during ageing: solid solution, work hardening, recovery and precipitation. Microstructure analyses revealed the formation of undissolved particles consuming the small Si addition, which influences age hardening behaviour of the alloys. Tensile testing was performed to evaluate the strength and work hardening. The integrated experimental results showed that for cold worked samples, b² (Mg2Si) contributes to age hardening of Cu-free alloys, whilst both b² and S (Al2CuMg) contribute to that of Cu-containing alloys. According to the experimental findings, a yield strength model has been developed to elucidate the relation between processing and the final strength. It consists of three main components: i) dissolution of intermetallic phase Mg2Si; ii) precipitation of two strengthening phases b² and S; iii) strengthening contributions from solution strengthening, dislocation strengthening and precipitation hardening due to the strengthening phases. The model was calibrated and tested using separate tensile data and was applied to predict the yield strength evolution of cold worked samples during ageing. An accuracy of 8.6 MPa (about 4% of the total range of strengths) has been achieved. Based on the analysis of the relation of work hardening with cold work, composition and ageing time, three primary findings were obtained: i) cold worked samples usually have the lowest work hardening rate (WHR); ii) WHR increases after 30-minute ageing due to recovery and iii) WHR increases with decreasing level of cold work and increasing Mg and Cu contents. Work hardening models based on the Kocks-Mecking (KM) model and the Kocks-Mecking-Estrin (KME) model have been utilized to explain the main trends. The modelling results showed that the KM model is able to predict the work hardening behaviour of cold worked samples reasonably well. However, the KME model is insufficient to fully describe that of cold-worked-and-aged samples.

The aim of this thesis is to investigate the influence of ageing process on the microstructure and mechanical properties of aluminium-silicon alloys. The investigation was carried on Al-9%Si-3%Cu and Al-9%Si-0.4%Mg. To obtain different DAS with low content of oxide films and micro shrinkage, gradient solidification has been used. The specimens were treated according to T6 heat treatment.

In this thesis it has been shown that solidification rate has great influence on mechanical properties since it controls microstructure. To reach peak level of mechanical properties different times of artificial ageing were used depending on the alloy.

In peak value condition Yield’s Strength of alloys was 197MPa for Al-Si-Cu alloy and 243MPa for Al-Si-Mg one. These results can be compared to these presented in other papers concerning aluminium silicon alloys. Such comparison shows that when talking about potential of alloy, these results are more or less the same as in other articles in this field.

The work was conducted within 10 weeks and for this reason not all the necessary data was collected. Further work will be conducted to obtain missing results, like overaged state for Al-Si-Cu alloy.

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension-compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with i) the precipitation structure and ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the precipitate average radius and is thus linked to the shearable/non shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually lead to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al, a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the precipitation structure main characteristics –such as the average precipitate radius, length and volume fraction– allows one to estimate accurately the internal stress contribution to the flow stress with this model.

碩士
國立中興大學
材料工程學系所
97
In order to seek for the top-class of “light”, lots of different materials’ “specific strength” is reviewed in the transport industry. So “Aluminum-alloy” applications are keeping growing up. Traditional forming-way are casting and forging. Due to that we can get more uniform structure and better fatigue strength and toughness via forging process, so it has been the first choice of the consideration. Unfortunately, “Coarse-Grains” issue keeps upset the forging industry, cause big problem on mechanical properties and surface-treatment .The forging industry still use “Try and Error” method on this issue, but people in other places have been having some different view points to try to overcome this problem.We try to control raw material quality and forging parameter to study this issue, will focus on 6061, 6066 and 7050. The three most popular forging aluminum alloys. First, we will try do some study about composition-comparison/electrical-conductivity/cross-section hardness/microstructure observation to select the suitable raw material rod. Later, we will change the parameter (billet temperature/billet heating time/tooling temperature/heat-treatment) to see the change about mechanical properties/grain size/electrical conductivity. We found that if we wish to get smaller grain, even both 6061 and 6066 are Al-Mg-Si alloy, the best billet temperature are not the same.6066’s “best point” is 60℃ higher than 6061. It is not like what the proverb say “the lower billet temperature, the smaller grain”. For 7050, annealed status of raw material will be a very important factor for final grain distribution of final forging parts. If we use “good” annealed status raw material, can get smaller and more uniform grain, raise the mechanical properties, and still keep very good toughness. In practical application, we can know precipitation status of forging parts via electrical conductivity, with mechanical properties, via proper adjusting manufacturing parameter, got the proper mechanical properties we need.

La présente étude a été réalisée en vue d'enquêter sur l'influence d'un certain nombre de paramètres métallurgiques sur les propriétés de traction et les indices de qualité de deux alliages de fonderie à haute résistance Al-9%Si, à savoir les alliages 354 et 359, contenant respectivement 1,8% Cu-0.5 % Mg et 0,5% Mg. Ces alliages ne sont que très peu utilisés par les fondeurs malgré le fait qu'ils sont prometteurs dans plusieurs domaines d'applications d'ingénierie en raison de leur résistance supérieure. Afin de valider leur utilisation dans des applications industrielles, une solide base de données a été créée grâce à la présente étude. De plus, une corrélation entre les propriétés de traction, les indices de qualité et les paramètres métallurgiques les plus communs affectant et contrôlant les propriétés de ces alliages a été réalisée. Les variables étudiées sont notamment le niveau de fer, la teneur en cuivre, le niveau de magnésium, la teneur en strontium, le taux de solidification, les températures et les temps des traitements thermiques (mise en solution et le vieillissement). Les chartes d'indice de qualité ont été utilisées comme outil d'évaluation pour sélectionner les conditions optimales à appliquer dans afin de développer une résistance élevée et une qualité optimale des alliages de fonderie 354 et 359. Une analyse des résultats montre que l'addition de fer a des effets délétères sur la force et la qualité des alliages 354 et 359. Ces effets sont liés à la taille et à la morphologie des phases contenant du fer, en particulier les phases jS-AlsFeSi et T-AlgMgsFeSié. L'addition de cuivre aux alliages de type 359 joue un rôle important dans l'amélioration de la résistance, cette amélioration se produit cependant au détriment de la ductilité et ainsi une faible influence sur l'indice de qualité est noté. L'ajout jusqu'à 0,6% de magnésium à l'alliage 359 améliore considérablement la résistance sans affecter l'indice de qualité. L'augmentation du niveau de Mg au-delà 0,6% se traduit par une légère augmentation de la résistance de l'alliage avec une réduction notable de la ductilité et de l'indice de qualité due à la formation d'une fraction volumique importante de la phase Pi-Al8Mg3FeSi6. Un taux de solidification rapide améliore les propriétés de traction et les indices de qualité des deux alliages 354 et 359. L'amélioration de ces propriétés des indices de qualité sont liés à l'amélioration des caractéristiques des microstructures qui accompagne le taux de solidification rapide, à savoir: le raffinement de la structure dendritique (c'est à dire une fine DAS), la modification des particules de silicium eutectique, la réduction du niveau de porosité et de la taille des pores, la réduction de la taille et de la fraction volumique des phases intermétalliques formées. Une augmentation de la température de mise en solution améliore la résistance et la qualité des pièces moulées par rapport à la condition telle que coulée. Conformément à ces constatations, les températures maximales sécuritaires de mise en solution de 520°C et de 537°C sont respectivement recommandées pour le traitement thermique de pièces moulées pour les alliages 354 et 359 afin de produire une résistance et un indice de qualité optimaux. Une mise en solution à ces températures produit un certain nombre des11 changements bénéfiques dans la microstructure ce qui se traduit par une amélioration importante de la résistance et de l'indice de qualité. Une mise en solution à des températures supérieures à 525°C entraîne la fonte naissante des phases AlaCu dans les alliages 354 et la formation d'un niveau élevé de porosité lors du retrait des pièces coulées après trempe. En conséquence de la fonte naissante, les particules de silicium dans la microstructure montrent une morphologie polygonale dans les régions où la fonte de la phase Al2Cu s'est produite. Une augmentation du temps de mise en solution améliore encore les propriétés de traction et les valeurs de l'indice de qualité des alliages 354 et 359. Douze heures a été jugé comme temps optimal pour le traitement thermique des pièces coulées. Le temps de mise en solution nécessaire pour obtenir des propriétés de traction et des indices de qualité spécifiques pour les alliages 354 et 359 non modifiés peut être raccourci en modifiant ces alliages avec le strontium. Toutefois, l'effet bénéfique de l'addition du Sr sur la réponse des alliages 354 et 359 diminue à mesure que le temps de mise en solution augmente. Un vieillissement à 155°C permet de produire une plus grande résistance et un indice de qualité optimal pour les deux alliages 354 et 359 par rapport à un vieillissement à des températures plus élevées. Le pic de résistance observée pour les alliages 354 et 359 peut être atteint après un temps de vieillissement plus court, à condition que la température de vieillissement soit accrue. Les temps de vieillissement nécessaire pour atteindre la résistance maximale pour l'alliage 354 sont de 72 heures, 40 heures, 8 heures, 1 heure et 15 minutes à des températures de vieillissement de 155°C, 170°C, 195°C, 220°C et 245°C respectivement, tandis que les temps de vieillissement nécessaires pour atteindre la résistance maximale dans l'alliage 359 sont de 32 heures, 24 heures, 1 heure, 30 minutes et 10 minutes respectivement, aux mêmes températures de vieillissement. Un traitement de vieillissement à haute température est accompagné d'une réduction des propriétés de traction et de la valeur de l'indice de qualité. D'autre part, ce traitement introduit aussi la possibilité de réaliser des gains économiques importants en diminuant le temps et le coût du traitement. Un vieillissement à basse température (Î55°C) produit des précipités fins et denses ayant un espacement plus petit entre les particules, tandis qu'à une température plus élevée (245 °C par exemple), les précipités sont moins denses, leur taille est plus grossière et ceux-ci sont dispersés plus largement dans la matrice. II a été observé que les alliages 354 ont affichés des niveaux de résistance plus élevés, comparé aux alliages 359 et ce, pour tous les traitements de vieillissement. Cependant, cette haute résistance a été obtenue au détriment de la ductilité, entraînant de légères variations dans les valeurs de l'indice de qualité des pièces coulées à partir de l'alliage 354. Les chartes des indices de qualité développées dans le cadre de cette recherche facilitent l'interprétation et l'évaluation des propriétés de traction des alliages 354 et 359 en vertu d'une variation des éléments d'alliage ajoutés, de la solidification et des conditions de traitement thermique étudiées. L'utilisation de ces chartes pour l'interprétation desm propriétés en traction fournit un moyen précis et logique pour déduire l'influence de ces paramètres métallurgiques sur ces alliages. Basée sur les chartes de qualité développées, il est possible de faire une sélection précise des conditions les plus appropriées requises pour obtenir le meilleur compromis possible entre la résistance d'un alliage, son indice de qualité et le coût de production.

Programa Doutoral em Engenharia de Materiais
The Al–Mg–Si–Cu family of alloys have been widely used in the automotive industry due to the high castability (excellent foundry characteristics) promoted by Si presence. The high strength to weight ratio and the good corrosion resistance are also important properties normally associated to Al alloys. Nevertheless, in general they possess low wear resistance. Typically, either age-hardening heat treatments or the addition of SiC ceramic particles to Al matrix are used to improve the mechanical properties of Al alloys. Further, by using centrifugal casting technique, the creation of a gradual decrease of the reinforcing particles volume fraction from the surface to the bulk, allows the conception of materials with high surface hardness and bulk toughness, this is functionally graded materials – FGM. Although the presence of SiCp can change the corrosion resistance of Al matrix composites, the excellent wear resistance of these materials, well recognized in the literature, make Al-SiCp composites candidates for tribocorrosion applications. In the present work, age-hardened unreinforced centrifuged Al–10Si–4.5Cu-2Mg (wt.%) alloys as well as age-hardened functionally graded Al-SiCp composites were produced by centrifugal casting. Their dry sliding wear, corrosion and tribocorrosion behaviour were studied in detail. The corrosion behaviour the Al–10Si–4.5Cu-2Mg alloy was governed by the galvanic coupling between the -Al2Cu phases and the -Al matrix. In static conditions, the behaviour of the alloys was characterized by active dissolution. The cathodic reactions were kinetically limited by water reduction as well as mass transport controlled oxygen reduction. In fact, under mass transport conditions, it was demonstrate that the oxygen reduction reaction could change from a 4-electron to a 2-electron process at high rotation rates (1600 rpm). This change was promoted by the small sized -Al2Cu phases in the age hardened alloys. Above this critical rotation rate value, the oxygen reduction rate was kinetically limited. The tribocorrosion behaviour of the unreinforced centrifuged Al alloy was studied in NaCl solution (typically aggressive solution to Al alloys) and in NaNO3 solution (inhibitor effect on the corrosion of Al alloys). The film properties of the Al alloys allowed the development of a simple galvanic coupling model that predicts the electrochemical response to sliding under OCP conditions. Regarding the functionally graded Al-SiCp composites, in dry sliding conditions, the SiC reinforcement particles content played a role in the wear response. In fact, SiC contents values lower than ≈ 5%, the wear coefficient rapidly increased. A further increase of volume fraction of SiC particles, above 5%, reflects lower wear coefficient values. Also, a relation was found between the amount of SiC reinforcing particles and the type of wear regime. No effect of SiC particles presence was observed on the corrosion behaviour of the composite materials. The age-hardening heat-treatments of the FGM’s does not have influence on their corrosion behaviour. Under tribocorrosion conditions, the critical SiC content was ≈18%. Above this critical value, it was observed SiC particles protrude from the surface and thus protecting the surrounding metal matrix against wear. Below the critical content, the SiC reinforcement had no effect on wear.
A família das ligas de Al-Si-Cu-Mg tem sido muito usada na indústria automóvel especialmente devido às excelentes características de fundição (presença de Si). A relação entre elevada resistência mecânica e baixa densidade assim como a boa resistência à corrosão são também propriedades importantes reconhecidas nas ligas de Al. No entanto, em termos gerais, estas ligas apresentam baixa resistência ao desgaste. Tipicamente, tanto os tratamentos térmicos de endurecimento por precipitação como a adição de partículas cerâmicas de elevada dureza são métodos usados para melhorar as propriedades mecânicas das ligas de Al. Adicionalmente, o uso de fundição centrífuga permite a criação de um decréscimo gradual da quantidade de partículas de reforço, desde a superfície do componente até ao seu interior, permitindo assim a criação de materiais com elevada dureza superficial e elevada tenacidade no interior, isto é, um material com gradiente funcional - “functionally graded materials, FGM”. Embora a presença de SiCp possa modificar a resistência à corrosão dos compósitos com matrizes de alumínio, a excelente resistência ao desgaste destes materiais, referenciada em diferentes bases bibliográficas, torna os compósitos de Al-SiCp candidatos para aplicações de tribocorrosão. No presente estudo, a liga Al–10Si–4.5Cu-2Mg (wt.%) e os compósitos de Al-SiCp com gradiente funcional foram produzidos por fundição centrifuga. Ambos foram tratados termicamente por envelhecimento artificial. Os seus comportamentos de desgaste a seco, corrosão e tribocorrosão foram estudados em detalhe. O comportamento à corrosão da liga Al–10Si–4.5Cu-2Mg é governado pela formação de pares galvânicos entre as fases -Al2Cu e a matriz -Al. Em condições estáticas, o comportamento das ligas é caracterizado por dissolução activa. As reacções catódicas foram identificadas como limitadas cineticamente pela reacção de redução da água assim como controladas, em condições de transporte de massa, pelas reacções de redução do oxigénio. Em condições de transporte de massa, com elevadas taxas de rotação (1600 rpm), foi demonstrado que a reacção de redução de oxigénio podia mudar de um processo que envolvia 4 electrões para um que envolvia 2 electrões. Esta mudança foi devida ao tamanho reduzido das fases de -Al2Cu nas ligas de Al envelhecidas por precipitação. Para valores de rotação superiores aos críticos, a taxa de redução do oxigénio passou a ser limitada cineticamente. O comportamento à tribocorrosão das ligas de Al (centrifugadas e sem partículas de reforço) foi estudado numa solução de NaCl (solução tipicamente agressiva para as ligas de Al) e numa solução de NaNO3 (inibidor de corrosão nas ligas de Al). As propriedades do filme presente na superfície das ligas de Al, permitiu o desenvolvimento de um modelo simples de pares galvânicos, que prevê a resposta electroquímica às solicitações mecânicas, em condições de potencial em circuito aberto (OCP). Relativamente aos compósitos de Al-SiCp com gradiente funcional, em condições de desgaste a seco, a quantidade de partículas de reforço mostrou ter influência no comportamento ao desgaste destes materiais. Foi observado um aumento rápido do coeficiente de desgaste dos compósitos FGM para quantidades de partículas de SiC inferiores a ≈ 5%. O coeficiente de desgaste diminui com o aumento na fracção volúmica de partículas de SiC acima de 5%. Também foi estabelecida uma relação entre a quantidade de partículas de reforço e o tipo de regime de desgaste. A presença das partículas de SiC não demonstrou ter qualquer efeito no comportamento à corrosão dos materiais compósitos. Os tratamentos térmicos de envelhecimento por precipitação executados aos materiais compósitos também não demonstraram ter influência neste ponto. Em condições de tribocorrosão, a quantidade crítica de partículas de SiC foi de ≈18%. Acima deste valor, foram observadas partículas sobressalientes na superfície do material compósito, protegendo a matriz metálica de desgaste mecânico. Para quantidades de partículas de SiC inferiores a este valor crítico, não foi demonstrada qualquer influência das partículas de reforço no desgaste do material.

碩士
國立成功大學
材料科學及工程學系
104
In this study, high Si-containing aluminum alloys synthesized by spray-formed are extruded. The author study the effects of different extrusion parameters on the microstructure, tensile properties and wear properties at elevated temperature. Establishing a database to develop cylinder liner for motorcycle engine with the results of tensile test and wear test at elevated temperature. Thermo-calc analysis is similar with microstructure of AC9A-30Si and AZFM-30Si synthesized by spray forming. Different extrusion parameters will influence microstructure of AC9A-30Si, AZFM-30Si. AC9A-30Si extruded at 500℃,0.1s-1 have highest peak stress, yield stress but lowest elongation. AZFM-30Si extruded at 500℃,0.1s-1 and etched have better wear resistance.

Over the last three decades the semi-solid metal (SSM) processing has received significant attention. Semi-solid processing involves the net shape manufacturing of alloys in the semi-solid state. The principal attraction for the process has been the unique rheology of the slurry which induces better movement of materials through the die and allows intricate thin-wall near net shape components to be cast at lower applied pressures. This behaviour offers considerable advantages to the quality of castings. The reduced oxide entrapment, low porosity and a lower operating temperature make semi-solid processing ideal for the forming of high integrity parts. The aim of the current study was to investigate the rheological behavior and microstructural evolution of hypereutectic Al-Si-Cu and Al-Si-Mg-Cu alloys using conventional and modified SEED process (Swirled Enthalpy Equilibration Device). This project is divided into four parts. In the first part, the feasibility of semi-solid processing of hypereutectic Al-Si-Cu A390 alloys using a novel rheoforming process was investigated. A combination of the SEED process, isothermal holding using insulation and addition of solid alloy during swirling was introduced as a novel method to improve the processability of semi-solid A390 slurries. The effects of isothermal holding and the addition of solid alloy on the temperature gradient between the centre and the wall and on the formation of a-Al particles were examined. In addition, phosphorus and strontium were added to the molten metal to refine the primary and eutectic silicon structure to facilitate semi-solid processing. It was found that the combination of the SEED process with two additional processing steps can produce semisolid 390 alloys that can be rheoformed. The microstructure reveals an adequate amount of non-dendritic a-Al globules surrounded by liquid, which greatly improves the processability of semi-solid A390 slurries. In the second part, the effects of Mg additions ranging from 6 to 15% on the solidification behaviour of hypereutectic Al-15Si-xMg-4Cu alloys was investigated using thermodynamic calculations, thermal analysis and extensive microstructural examination. The Mg level strongly influenced the microstructural evolution of the primary Mg2Si phase as well as the solidification behaviour. Thermodynamic predictions using ThermoCalc software reported the occurrence of six reactions, comprising the formation of primary Mg2Si, two pre-eutectic binary reactions, forming either Mg2Si + Si or Mg2Si + a-Al phases, the main ternary eutectic reaction forming Mg2Si + Si + a-Al, and two post-eutectic reactions resulting in the precipitation of the Q-Al5Mg8Cu2Si6 and O-Al2Cu phases, respectively. Microstructures of the four alloys studied confirmed the presence of these phases, in addition to that of the 7i-AlgMg3FeSi6 phase. The presence of the pi-phase was also confirmed by thermal analysis. The morphology of the primary Mg2Si phase changed from an octahedral to a dendrite form at 12.52% Mg. Further Mg addition only coarsened the dendrites. Image analysis measurements revealed a close correlation between measured and calculated phase fractions of the primary Mg2Si and Si phases. ThermoCalc and Scheil calculations show good agreement with the experimental results obtained from microstructural and thermal analyses. In the third part, the effects of P and Sr on the microstructure of hypereutectic Al-15Si-14Mg-4Cu alloy were studied. The microstructural examination and phase identification were carried out using optical microscopy and scanning electron microscopy (SEM). The effects of individual and combined additions of P and Sr on the eutectic arrest in Al-15Si-14Mg-4Cu alloy were examined using thermal analysis. The mean size of primary Mg2Si decreases from about 350 um to less than 60 um and the morphology changes from coarse dendritic type or equiaxed to polygonal type. In addition, the morphology of the eutectic Mg2Si phase changes from coarse Chinese script to fine fiber-like, while that of the eutectic Si phase changes from coarse acicular shape to a fine fibrous form. With Sr addition, the morphology of the pi-Fe phase evolved from Chinese script to a fine twin platelet form. Furthermore, the thermal analysis results reveal that the addition of Sr or Sr and P reduces the temperature of eutectic nucleation and growth. Quantitative measurements revealed a reduction in the particle area and an increase of the density of Mg2Si, Si and pi-Fe phases. The Sr or Sr + P combined additions are effective in modifying the eutectic Si, Mg2Si and pi-Fe phases for the Al-15 Si-14Mg-4Cu alloy. Finally, the rheological behaviour and microstructure of semi-solid hypereutectic A390, P-refined A390, Al-15Si-10.5Mg-4Cu and Al-15Si-13.5Mg-4Cu alloys were investigated by using parallel plate viscometry. The flow deformation of these alloys in the semi-solid state was characterized at different deformation rates and at variable solid fractions. The calculated viscosity for variable shear rate was deduced using the analytical method developed by Laxmanan and Flemings. Microstructures of the four alloys, after partial solidification, were examined in order to characterize the flow behaviour during deformation. An image analysis was used for quantification of particle segregation and effective volume fraction. The apparent viscosity of all studied alloys increased with increasing solid volume fraction, and decreased with increasing shear rate. The comparison of the apparent viscosity of the four alloys indicated that the higher the Mg in the alloy, the higher the apparent viscosity was for the range of shear rates and solid fractions investigated. It is also shown that the refined A390 alloy has the lowest apparent viscosity due to the small size of primary Si particles. In addition, a separation of liquid and solid phase was also observed for all alloys in the microstructure study of deformed semi-solid billets. - Au cours des trois derrières décennies, la mise en forme à l'état semi-solide a suscité beaucoup d'attention. Cette méthode consiste à fabriquer des pièces mécaniques finies à l'état semi solide. L'attraction principale de cette technique réside essentiellement dans son unique propriété rhéologique qui confère un meilleur écoulement à la gelée à travers la filière du moule et permet par la suite l'optimisation de la pression nécessaire pour un remplissage adéquat. L'objectif principal de ce travail est l'élude du comportement rhéologique ainsi que l'évolution microstructurale des alliages hypereutectiques Al-Si-Mg-Cu en utilisant deux méthodes différentes, le procédé SEED conventionnel et modifié. Ce travail de recherche est divisé en quatre parties. La première partie est consacrée à l'étude de la faisabilité de la mise en forme à l'état semi-solide d'alliage hypereutectique Al-Si-Cu 390 en utilisant un nouveau procédé de rhéoformage. En effet, la combinaison du procédé SEED conventionnel, un maintien isotherme avec isolation du moule et l'ajout de petits morceaux d'alliages pendant le brassage a été introduit comme une nouvelle méthode capable de produire une gelée. Par la suite, l'effet induit par le maintien isotherme et l'ajout de morceaux d'alliages sur le gradient de température au centre et à la paroi du moule ainsi que sur la formation des particules a-Al a été examiné. Dans certain tests supplémentaires, des quantités appropriées de phosphore et de strontium ont été introduite dans le métal liquide dans le but d'affiner le silicium primaire et eutectique, et faciliter par la suite la mise en forme de la gelée. Il a été constaté que la combinaison du procédé SEED avec deux étapes supplémentaires est une technique capable de produire une gelée d'alliage A390. En plus, la microstructure de la gelée révèle une quantité suffisante de globules d'aluminium (a-Al) entourées de liquide. Dans la deuxième partie, l'effet de l'addition de quantités de Mg allant de 6 à 15% sur le chemin de solidification et la microstructure des alliages hypereutectiques Al-15Si-xMg-4Cu a été étudié en utilisant des calculs thermodynamiques, une analyse thermique et un examen microstructural approfondi. La teneur en Mg influence fortement la cinétique de formation de la phase primaire Mg2Si ainsi que le chemin de solidification du système. La prédiction thermodynamique en utilisant ThermoCalc montre l'existence de six réactions, incluant la formation de la phase primaire Mg2Si, deux réactions pré-eutectique binaires formant soit les phases Mg2Si et Si ou Mg2Si et a-Al, la réaction eutectique ternaire (Mg2Si + Si + a-Al), et deux réactions post-eutectiques aboutissant à la précipitation des phases QAl5Mg8Cu2Si6 et O-Al2Cu, respectivement. Les microstructures des quatre alliages étudiés ont par la suite confirmé la présence de toutes ces phases, y compris la phase nIV Al8Mg3FeSi6. La présence de la phase pi-Fe riche en fer a été également confirmée par l'analyse thermique. L'addition de teneurs supérieures à 12.52% Mg induit une évolution de la morphologie de la phase primaire Mg2Si d'une forme octaédrique vers une forme dendritique et une augmentation significative des dendrites. L'analyse quantitative a révélé une corrélation entre les fractions volumiques mesurées et calculées de la phase primaire Mg2Si et Si. Enfin, les résultats obtenues par ThermoCalc et soutenues par la méthode Scheil montrent un bon accord avec les résultats expérimentaux obtenus à partir des analyses microstructuraux et thermiques. Dans la troisième partie, l'effet des éléments d'addition P et Sr sur la microstructure de l'alliage hypereutectique Al-15Si-14Mg-4Cu a été étudié. La caractérisation microstructural et l'identification des différentes phases ont été réalisées en utilisant un microscope optique et un microscope électronique à balayage (MEB). L'apport individuel et combiné de P et Sr sur la température eutectique de l'alliage Al-15Si-14Mg-4Cu a été étudié à l'aide de l'analyse thermique. La taille moyenne de la phase primaire Mg2Si a diminuée de 350 um à moins de 60 um et sa morphologie a évoluée d'une forme dendritique vers une forme polygonale. En plus, la morphologie des phases eutectique Mg2Si et Si ont changé respectivement d'une forme d'écriture chinoise et d'une forme aciculaire vers une forme fibreuse de taille fine. L'addition de Sr a aussi montré le changement de la morphologie de la phase intermétallique 7t-Fe. Les résultats de l'analyse thermique ont révélés une diminution des températures de germination et de croissance eutectique. L'analyse quantitative a montrée une réduction de la taille des particules et l'augmentation de la densité des phases Mg2Si, Si et rc-Fe. L'addition de Sr ou la combinaison de Sr avec P est avéré très efficace pour affiner la phase primaire Mg2Si et modifier les phases eutectiques Mg2Si, Si ainsi que la phase rc-Fe de l'alliage Al-15Si-14Mg-4Cu. Enfin, le comportement rhéologique et l'évolution microstructural des alliages hypereutectique A390, A390 affiné, Al-15Si-10.5Mg-4Cu et Al-15Si-13.5Mg-4Cu à l'état semi-solide ont été étudiés à l'aide d'un viscosimètre. La déformation de ces quatre alliages à l'état semi-solide a été caractérisée à différentes vitesses de déformations et à fractions de solides variables. La viscosité apparente a été calculée en utilisant le modèle développé par Laxmanan et Flemings. Les microstructures des quatre alliages, après solidification partielle, ont été examinées afin de caractériser le comportement rhéologique lors de la déformation. Une analyse d'image a été réalisée pour quantifier la ségrégation des particules solides et la fraction volumique effective. Les résultats montrent une augmentation de la viscosité apparente des quatre alliages étudiés avec l'augmentation de la fraction solide, et la diminution du taux de cisaillement. La comparaison de la viscosité apparente des quatre alliages indiquait que l'alliage contenant une teneur élevée en Mg possédait une plus grande viscosité apparente pour la gamme du taux de cisaillement et de fractions solides étudiés. Il est également montré que l'alliage affiné A390 a la plus faible viscosité apparente en raison de la diminution de la taille des particules de silicium primaire. En outre, une séparation des phases liquides et solides a également été observée pour tous les alliages au cours de l'étude microstructural des billettes déformées à l'état semi-solide.

Aluminum-silicon casting alloys have shown great promise in several fields of engineering and have proved highly suitable for use in automotive, marine, and aerospace applications where, through appropriate molten metal processing and suitable heat treatments, the manufacturing of diverse parts is made possible. Developments in aluminum alloys and optimization of casting techniques have led to improved material properties and functional integration which enable aluminum castings to satisfy current market requirements. Although the automotive industry is focused on reducing fuel consumption, the development of diesel and direct fuel injection gasoline engines with high specific powers in recent years have resulted in a marked performance impact on piston materials due to increased combustion pressure and piston temperatures. Reductions in the weight of key engine components together with higher service temperatures would allow for more efficient operation. While aluminum-silicon cast components are generally limited to service conditions of no more than 230°C, operating conditions in automotive components often result in much higher temperatures. Improving the high temperature mechanical properties requires an understanding of the factors which are related principally to a decrease in the strength of the alloy with increasing temperature. The use of transitions element additions of Ni, Zr and Sc to increase high temperature strength in aluminum alloys is based on the production of coherent or semi-coherent L12 type precipitates of Al3Ni, Al3Zr, or Al3Sc, which are better able to support overaging at higher temperatures due to their stability at higher temperatures, compared to the Al2Cu and Mg2Si precipitates normally present in cast aluminum alloys. The present study was carried out to investigate the effects of Ni (high and low), Mn, Zr, and Sc additions, individually or in combination with other additives, on the microstructure and tensile properties of 354 casting alloy (Al-9wt%Si-1.8wt%Cu-0.5wt%Mg) at ambient and at high temperatures (155°C and 300°C) using different holding times at testing temperature. Tensile tests were carried out in the as-cast, solution heat-treated, and aged conditions using different aging temperatures and times. Quality charts were used as an evaluation tool for selecting the optimum conditions to achieve superior tensile properties and optimum quality in 354-type alloys. Nine alloys were prepared using the 354 alloy, comprising a) Stage I alloys – base 354 alloy (G1) and four others (alloys G2 to G5) containing Ni (low) and Zr additions, for testing at room and at high temperature, and b) Stage II alloys comprising the base alloy (G1) and five others (alloys G6 to G10) containing Ni (high), Mn, Sc, Zr, added individually or in combination, and one alloy (G7) containing no copper, which were tested at room temperature. An analysis of the data obtained from microstructural and thermal analyses and tensile tests shows that the tensile behavior of 354-type cast alloys is strongly influenced by the testing temperature and the holding time at temperature prior to testing. The influence of minor additions of Ni and Zr to these alloys on their high temperature performance is controlled by their T6-properties at room temperature. Addition of 0.2wt% Ni and 0.2wt% Zr (alloy G4) improves the T6-tensile properties considerably, compared to the as-cast condition. Addition of 0.4wt% Ni + 0.4wt% Zr (alloy G5) is not sufficient to resist softening at 300°C/100h. Addition of 0.4wt% Ni to alloy 354 (G2 and G5 alloys) leads to a decrease in the tensile properties, attributed to a Ni–Cu reaction that interferes with the formation of Al2Cu strengthening precipitates and affects the age hardening process. The L12 Al3(Zr,Ti) phase, the block-like (Al,Si)2(Zr,Ti) phase, and the needle-like (Al,Si)3(Zr,Ti) phase are the main features observed in the microstructure of alloys G3, G4 and G5 containing 0.2-0.4wt% Zr additions. The presence of Q-Al5Cu2Mg8Si6 phase and Al3Ni phase is observed in samples tested at 300°C after 10 h holding. Holding the tensile samples of T6-treated G1 alloy resulted in a marked increase in the density of precipitated particles, most of the particles maintaining a spherical shape. Also, aging the solutionized alloy at 190°C for 2 h resulted in an increase in the density of the precipitated Al2Cu. Increasing the holding time prior to deformation to 100 h resulted in the change in the morphology of the Al2Cu precipitates into thin platelets distributed in two perpendicular directions. In the Stage II alloys, the main reactions are detected during the solidification of the 354 alloys: formation of the α-Al dendritic network, followed by precipitation of the Al-Si eutectic and post-eutectic β-Al5FeSi; Mg2Si phase; transformation of the β-phase into π-Al8Mg3FeSi6 phase; and finally precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 phases. With 2wt% Ni addition, the formation of Al9FeNi and Al3CuNi phases is observed. In the base 354 alloy, the main phases are restricted to Cu-, Mg-, and Fe-rich intermetallic phases. The Si particle characteristics and volume fraction of intermetallics are influenced by the cooling rate and Mg level, while addition of Fe and/or Mn also has a slight influence (alloy G9). In alloys G8 and G9, Fe, Mn and Ni interact to form new intermetallic phases; an increase in Fe content leads to the formation of polyhedral/star-like sludge particles in addition to the α-Fe script and β-Al5FeSi phases, increasing the volume fraction of intermetallics formed; the presence of the hard sludge particles within the soft α-Al dendrites improves the alloy properties. The secondary dendrite arm spacing (SDAS) does not differ much among the alloys studied. The average SDAS of the as-cast tensile samples of the six alloys was found to be ~18 μm. Solution treatment of Stage II alloys (at 500°C/8h) results in almost complete dissolution of the Al2Cu phase; partial dissolution of the β-Al5FeSi phase, and a high degree of decomposition of the π-phase into β-phase; the Q-Al5Mg8Cu2Si6, α-Fe and sludge particles remain insoluble. Zirconium and Sc react only with Ti, Si and Al to form ZrSi, (Al,Si)2(Zr,Ti), (Al,Si)3(Zr,Ti), Si(Ti,Zr), Al3Zr, and Si(Sc,Zr) phases. The Zr-rich intemetallics appear in two different forms: (a) the block-like (Al,Si)2(Zr,Ti) phase which contains a higher level of Si, and the needle-like (Al,Si)3(Zr,Ti) phase containing a higher level of aluminum. The beneficial effects of Zr, Sc and Ti additions appear in the refining of the α-Al grain size and transforming its morphology from dendritic to non-dendritic type, which reduces the size of the Al2Cu and α-Fe particles. The Zr-containing intermetallics appear to be refined and more uniformly distributed in the matrix in the presence of Sc. Among the Stage II alloys (G6 to G10), solution heat treatment improves the tensile properties and alloy quality of all alloys. The presence of Ni and Zr in G7 alloy (with no Cu), and Sc and Zr in G10 alloy provides further improvement as these alloys show the best Q and YS values. With respect to the addition of Cu and Mg in 354 type alloys, the quality index of the alloy castings is controlled by the net amount by which the strength is increased and the ductility is reduced. Aging treatment of 354-type alloy castings produces a wide range of tensile properties based on the aging temperatures and times applied. Multiple aging peaks are observed in the age-hardening curves of alloys G6 to G10, related to the precipitation sequence which occurs in each alloy. Depending on the required tensile properties, suitable aging conditions may be recommended based on the best possible compromise between strength, quality, and aging time involved in the process. The best combination of properties is achieved after aging at 190°C/2h for all alloys studied, with an exception of alloy G9 that showed the best combination at 190°C/4h. These conditions may therefore be considered as the appropriate T6 treatment for these alloys. Aging at 190°C introduces a technologically useful strategy for this particular alloy system as it provides a significant economic benefit in the form of a noticeable reduction in the aging time required to reach peak strength. The quality charts developed in the course of this study facilitate the interpretation of the tensile properties of 354 alloys, providing a logical evaluation tool for an accurate prediction of the influence of the various metallurgical parameters investigated on the alloys. Based on the quality charts developed, it is possible to make a rigorous selection as to the most suitable parameters to be applied to 354 type alloys so as to obtain the best possible cost-effective compromise between alloy strength and quality. Les alliages d’aluminium-silicium ont montré de nombreux avantages dans divers domaines de l’ingénierie. Ils ont prouvé être adaptés à l’industrie automobile, marine et aérospatiale pour lesquelles, avec des procédés de fusion et de traitements thermiques adaptés, la fabrication de différentes pièces est possible. Le développement des alliages d’aluminium et l’optimisation des techniques de coulées ont permis d’améliorer les propriétés des alliages et leur intégration fonctionnelle. Ceci permet aux alliages d’aluminium de satisfaire les critères actuels du marché. Bien que l’industrie automobile soit concentrée sur la réduction de la consommation d’essence, le développement ces dernières années des moteurs de haute puissance, à injection direct ainsi qu’à diesel, a entrainé un impact marqué sur la performance des matériaux servant à la fabrication des pistons, ceci due à l’augmentation de la pression de combustion et de la température des pistons. La réduction de poids dans les composants clés des moteurs ainsi qu’une température de service plus haute permettraient des opérations encore plus efficaces. Alors que les pièces coulées d’aluminium-silicium sont généralement limitées à des conditions de services inférieurs à 230°C, les conditions d’opérations des composants automobiles sont souvent à plus haute température. Améliorer les propriétés mécaniques de haute température demande une compréhension des facteurs reliés à une baisse de la résistance des alliages lors de l’augmentation de la température. L’ajout d’éléments de transition de Ni, Zr et Sc pour améliorer la résistance aux hautes températures des alliages d’aluminium est basé sur la production de précipité de Al3Ni, Al3Zr, ou Al3Sc de type L12 cohérent et semi-cohérent, lesquels supportent mieux le vieillissement à haute température, ceci due à une meilleure stabilité à ces mêmes températures en comparaison avec les précipités de Al2Cu et Mg2Si qui sont généralement présents dans les alliages d’aluminium. Cette étude a été réalisée dans le but d’évaluer les effets des ajouts de Ni (à fort ou faible taux), Mn, Zr et Sc, individuellement ou combinés avec d’autres additifs, sur les microstructures et les propriétés de traction, à température ambiante et à haute température, de l’alliage 354 (Al-9wt%Si-1.8wt%Cu-0.5wt%Mg) pour un temps de vieillissement plus ou moins long. Les échantillons de traction ont été testés dans des conditions «as-cast», et en utilisant diverses températures de traitement thermique et différents temps de vieillissement. Des diagrammes d’indice de qualité ont été utilisés comme outil d’évaluation pour choisir les conditions optimums devant être appliquées pour obtenir des propriétés de traction supérieures et la meilleure qualité possible pour les alliages 354. Neuf alliages 354 ont été préparés comprenant a) les alliages de l’étape I – alliage 354 de base (G1) et quatre autres (alliages G2 à G5) contenant des ajouts de Ni (faible taux) et de Zr, pour être testé à température ambiante et à haute température et b) les alliages de l’étape II contenant l’alliage de base (G1) et cinq autres (alliages G6 à G10) contenant Ni (fort taux), Mn, Sc, et Zr ajoutés individuellement ou combinés et un alliage (G7) ne contenant pas de cuivre, testé à température ambiante. Une analyse des données obtenues à partir des études microstructurale et thermique ainsi que des essais de traction montrent que le comportement mécanique des alliages 354 est fortement influencé par la température et le temps pendant lequel elle est maintenue avant l’essai. L’influence d’ajout mineur de Ni et Zr à ces alliages sur leur performance à haute température est contrôlée par leurs propriétés T6 à température ambiante. L’ajout de 0.2wt% Ni et 0.2wt% Zr (alliage G4) améliore considérablement les propriétés de traction T6 en comparaison au «as-cast». L’ajout de 0.4wt% Ni + 0.4wt% Zr (alliage G5) à l’alliage 354 n’est pas suffisant pour résister au ramollissement à 300°C/100h. L’ajout de 0.4wt% Ni à l’alliage 354 (alliages G2 et G5) amène une diminution des propriétés de traction. Cette diminution peut être attribuée à une réaction Ni-Cu qui interférerait avec la formation du précipité, renforçant Al2Cu, et affecterait le processus de durcissement par le vieillissement. La phase L12 Al3(Zr,Ti), la phase massive «block-like» (Al,Si)2(Zr,Ti), et la phase aciculaire (Al,Si)3(Zr,Ti) sont les principales caractéristiques observées dans la microstructure des alliages G3, G4 et G5 contenant 0.2-0.4wt% d’ajout de Zr. La présence de la phase Q-Al5Cu2Mg8Si6 et de la phase Al3Ni est observée dans les échantillons traités à 300°C et après plus de 10h de vieillissement. Maintenir pour un temps donné les échantillons de traction de l’alliage G1 traité thermiquement avec la méthode T6 entraine une augmentation marquée de la densité de particules précipitées, la majorité des particules restant de forme sphérique. De plus, vieillir l’alliage mis en solution à 190°C durant deux heures augmente la densité d’Al2Cu précipité. Augmenter la durée du temps de maintien, avant la déformation, à 100h permet de changer la morphologie du précipité Al2Cu en fines plaquettes distribuées selon deux directions perpendiculaires. Dans les alliages de l’étape II, les principales réactions sont détectées durant la solidification de l’alliage 354 soit: la formation de réseau dendritique de α-Al, suivi de la précipitation de des phases eutectique Al-Si et post-eutectique β-Al5FeSi; Mg2Si; la transformation de la phase β en π-Al8Mg3FeSi6; et finalement, la précipitation des phases Al2Cu et Q-Al5Mg8Cu2Si6. Avec un ajout de 2wt% Ni, la formation des phases Al9FeNi et Al3CuNi est observée. Dans l’alliage 354 de base, les principales phases sont restreintes aux phases intermétalliques de Cu, Mg et Fe. Les caractéristiques de la particule Si et le volume de fractions intermétalliques sont influencés par la vitesse de refroidissement et le taux de Mg, alors que l’ajout de Fe et/ou Mn à une faible influence (alliage G9). Dans l’alliage G8 et G9, Fe, Mn, et Ni interagissent pour former de nouvelles phases intermétalliques; une augmentation de la quantité de Fe entraine la formation de particules «sludge» de forme polyédrique et étoilée en plus des phases α-Fe et β-Al5FeSi, augmentant le volume de la fraction intermétallique formée; la présence de particules «sludge» dures dans les dendrites α-Al molles améliore les propriétés de l’alliage. Le deuxième espace du bras dendritique (SDAS) n’est pas très différent dans les divers alliages étudiés. La moyenne SDAS des échantillons de traction « as-cast » des six alliages est d’environ 18 μm. Le traitement en solution des alliages de l’étape II (à 500°C/8h) permet la dissolution presque complète de la phase Al2Cu; la dissolution partielle de la phase β-Al5FeSi, et un fort degré de décomposition de la phase π en phase β; les particules Q-Al5Mg8Cu2Si6, α-Fe et «sludge» restent insoluble. Zr et Sc réagissent uniquement avec Ti, Si et Al pour former les phases ZrSi, (Al,Si)2(Zr,Ti), (Al,Si)3(Zr,Ti), Si(Ti,Zr), Al3Zr, et Si(Sc,Zr). Les phases intermétalliques riche en Zr apparaissent sous deux formes différentes: (a) la phase (Al,Si)2(Zr,Ti) en forme de bloc qui contient plus de silicium, et la phase aciculaire (Al,Si)3(Zr,Ti) contenant plus d’aluminium. Les effets bénéfiques des ajouts de Zr, Sc et Ti se voit à l’affinement de la taille des grains α-Al et à la transformation de leurs morphologie dendritique en non dendritique, ce qui réduit la taille des particules Al2Cu et α-Fe. Les phases intermétalliques contenant Zr semblent être affinées et distribuées de manière plus uniforme dans la matrice en présence de Sc. Dans les alliages de l’étape II (G6 à G10), le traitement thermique améliore les propriétés de traction et la qualité de tous les alliages. La présence de Ni et Zr dans l’alliage G7 (sans Cu) et de Sc et Zr dans l’alliage G10 améliore ces alliages puisqu’ils montrent les meilleures valeurs Q et YS. Concernant l’ajout de Cu et Mg dans l’alliage 354, l’indice de qualité de l’alliage coulé est contrôlé par le taux d’augmentation de la résistance et de la réduction de la ductilité. Le vieillissement des alliages 354 coulés produit un large éventail de propriétés de traction dépendamment des températures et temps de vieillissement. Des pics multiples de vieillissement sont observés le long des courbes de durcissement par le vieillissement des alliages G6 à G10. Ces pics sont reliés à la séquence de précipitation qui a lieu dans chaque alliage. Dépendamment des propriétés de tension requises, des conditions de vieillissement appropriées peuvent être recommandées, basé sur le meilleur compromis entre la résistance, la qualité et le temps de vieillissement impliqués dans le procédé. La meilleure combinaison de propriétés est atteinte après un vieillissement à 190°C/h pour tous les alliages étudiés à l’exception de l’alliage G9 qui montre une meilleure combinaison à 190°C/4h. Ces conditions peuvent donc être considérées comme le traitement T6 le plus approprié pour ces alliages. Un vieillissement à 190°C est une méthode intéressante pour ce système d’alliage en particulier puisqu’elle amène une économie significative par la réduction du temps de vieillissement requis pour atteindre le pic de résistance. Les diagrammes d’indice de qualité développés durant ce projet facilitent l’interprétation des propriétés de traction de l’alliage 354, permettant un outil d’évaluation pour une prédiction précise de l’influence des paramètres métallurgiques étudiés pour ces alliages. Basé sur les diagrammes d’indice de qualité développés, il est possible de faire une sélection rigoureuse des paramètres les plus appropriés devant être appliqués aux alliages 354 et d’obtenir le meilleur compromis entre la résistance et la qualité de l’alliage.

In recent years, aluminum and aluminum alloys have been widely used in automotive and aerospace industries. Among the most commonly used cast aluminum alloys are those belonging to the Al-Si system. Due to their mechanical properties, light weight, excellent castability and corrosion resistance, these alloys are primarily used in engineering and in automotive applications. The more aluminum is used in the production of a vehicle, the less the weight of the vehicle, and the less fuel it consumes, thereby reducing the amount of harmful emissions into the atmosphere. The principal alloying elements in Al-Si alloys, in addition to silicon, are magnesium and copper which, through the formation of Al2Cu and Mg2Si precipitates, improve the alloy strength via precipitation hardening following heat treatment. However, most Al-Si alloys are not suitable for high temperature applications because their tensile and fatigue strengths are not as high as desired in the temperature range 230-350°C, which are the temperatures that are often attained in automotive engine components under actual service conditions. The main challenge lies in the fact that the strength of heat-treatable cast aluminum alloys decreases at temperatures above ~200°C. The Mg2Si and Al2Cu precipitates that maintain the high strength of the alloy usually coarsen or dissolve at temperatures above 150°C, resulting in reduced high temperature performance and consequently limited practical applications. Most Al-Si cast alloys to date are intended for applications at temperatures no higher than about 230°C. The strength of alloys under high temperature conditions is improved by obtaining a microstructure containing thermally stable and coarsening-resistant intermetallics, which may be achieved with the addition of Ni. Zr and Sc. Nickel leads to the formation of nickel aluminide Al3Ni and Al9FeNi in the presence of iron, while zirconium forms Al3Zr. These intermetallics improve the high temperature strength of Al–Si alloys. Some interesting improvements have been achieved by modifying the composition of the base alloy with additions of Mn, resulting in an increase in strength and ductility at both room and high temperatures. The principle effects that can be obtained by adding scandium to aluminium alloys are grain refinement and precipitation hardening from Al3Sc particles. Addition of Zr together with Sc is found to improve alloy strength and coarsening resistance. Al-Si-Cu-Mg alloys such as the 354 (Al-9wt%Si-1.8wt%Cu-0.5wt%Mg) alloys show a greater response to heat treatment as a result of the presence of both Mg and Cu. These alloy types display excellent strength values at both low and high temperatures. Additions of Zr, Ni, Mn and Sc would be expected to maintain the performance of these alloys at still higher temperatures. The present study was thus carried out to investigate the effect of Zr, Ni, Mn, and Sc additions, individually or in combination, on the microstructure and tensile properties of 354 casting alloy at ambient and at high temperature (250°C) using different holding times at testing temperature. Six alloys were prepared using 0.2 wt% Ti grain-refined 354 alloy, comprising alloy R (354 + 0.25wt% Zr) considered as the base or reference alloy, and five others, viz., alloys S, T, U, V, and Z containing various amounts of Ni, Mn, Sc and Zr, added individually or in combination. For comparison purposes, another alloy L was prepared from 398 (Al-16%Si) alloy, reported to give excellent high temperature properties, to which the same levels of Zr and Sc additions were made, as in alloy Z. Tensile test bars were prepared from the different 354 alloys using an ASTM B-108 permanent mold. The test bars were solution heat treated using a one-step or a multi-step solution heat treatment, followed by quenching in warm water, and then artificial aging employing different aging treatments (T5, T6, T62 and T7). The one-step (or SHT 1) solution treatment consisted of 5 h @ 495 °C) and the multi-step (or SHT 2) solution treatment comprised 5 h @ 495°C + 2 h @ 515°C + 2 h @ 530°C. Tensile testing of the as-cast and heat-treated test bars was carried out at room temperature using a strain rate of 4 x 10-4s-1. Five test bars were used per alloy composition/condition. In this case, the test bars were tested with or without prior stabilization for 200 h at 250 °C. The high temperature tensile testing was carried out at 250 °C, where the test bars were stabilized for 1 h and 200 h at 250 °C prior to testing. Thermal analysis of the various 354 alloy melts was carried out to determine the sequence of reactions and phases formed during solidification under close-to-equilibrium cooling conditions. The main reactions observed comprised formation of the α-Al dendritic network at 598°C followed by precipitation of the Al-Si eutectic and post-eutectic β-Al5FeSi phase at 560°C; Mg2Si phase and transformation of the β-phase into π-Al8Mg3FeSi6 phase at 540°C and 525°C; and lastly, precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 almost simultaneously at 498°C and 488°C. As a result of the low solidification rate of the thermal analysis castings, and a Zr content of 0.25 wt%, all Zrcontaining alloys are located in the L + Al3Zr region of the Al-Zr phase diagram during the melting stage. Three main reactions are detected with the addition of Ni, i.e., the formation of AlFeNi, AlCuNi and AlSiNiZr phases. Larger sizes of AlFeNi and AlCuNi phase particles were observed in T alloy with its higher Ni content of 4 wt%, when compared to those seen in S alloy at 2% Ni content. Mn addition in Alloy U helps in reducing the detrimental effect of the β-iron phase by replacing it with the less-detrimental Chinese script α-Al15(Fe,Mn)3Si2 phase and sludge particles. The Sc-intermetallic phases observed in this study appeared in two different forms: (Al,Ti)(Sc,Zr) and (Al,Si)(Sc,Zr,Ti). With the use of the multi-step solution treatment – involving higher solution temperatures and longer durations, an increased amount of incipient melting is expected to occur. Coarsening of the Si particles is also observed; with larger particles growing bigger at the expense of smaller ones. Primary Si particles are observed in the microstructure of the hypereutectic alloy L with its high Si content of 16 wt%. The tensile data showed that UTS and percent elongation of R, S, T, U, V and Z alloys increased in the one-step solution heat-treated condition compared to the as-cast case. The multistep solution heat treatment displayed higher tensile properties than those achieved with SHT 1 treatment. The use of the T62 treatment, incorporating the SHT 2, allows for maximum dissolution of the copper phases in the multiple stages of solution treatment, resulting in the greatest improvement in both UTS and YS. Without stabilization, T6 and T62 treatments provide the best improvements in both UTS and YS values of all alloys. The best tensile properties of alloys tested at room temperature after stabilization at 250°C for 200 h are obtained with the T6 heat treatment. After T62 treatment, Alloy U (containing 0.75wt% Mn + 0.25wt% Zr) showed the maximum increase in UTS and YS values. The addition of Zr, Ni, Mn and Sc to Al-Si alloys improves the high temperature tensile properties of the 354 alloy. Alloy S (Al 354 + 0.25wt% Zr + 2wt% Ni) and alloy U (Al 354 + 0.25wt% Zr + 2wt% Ni) perform better in case of high temperature conditions, with one hour stabilization at 250°C. After 200 hours stabilization at 250°C, the strength of the T6-treated alloys is reduced considerably, while the ductility is increased, with alloy R showing the highest percent elongation, ~19%, followed by Z alloy with a ductility of ~16%. The reduction in strength may be attributed to the alloy softening which occurs after such long stabilization time at the high testing temperature. Although the T6-treated L alloy displays good strength values, in comparison with the 354 alloys at the same high temperature testing conditions, its ductility is ~2.45% compared to alloy S which produces similar tensile strength but has a ductility of ~6.5%. The reduction in strength may be attributed to the alloy softening which occurs after such long stabilization time at the high testing temperature. Au cours des dernières années, l'aluminium et les alliages d'aluminium ont été largement utilisés dans l’industrie de l'automobile et de l'aérospatiale. Parmi les alliages d'aluminium coulé les plus couramment utilisés, figurent ceux appartenant au système Al-Si. En raison de leurs propriétés mécaniques, de leur légèreté, de leur excellente coulabilité et de leur résistance à la corrosion, ces alliages sont principalement utilisés dans l'ingénierie et dans les applications automobiles. Plus l'aluminium est utilisé dans la production d'un véhicule, plus le poids de celui-ci est diminué et contribue à diminuer sa consommation de carburant, réduisant ainsi la quantité d'émissions nocives dans l'atmosphère. Les principaux éléments d'alliage dans les alliages Al-Si, en plus du silicium, sont le magnésium et le cuivre. Ceux-ci améliorent la résistance à l'alliage par le phénomène de durcissement par précipitation suite au traitement thermique, grâce à la formation de précipités Al2Cu et Mg2Si. Cependant, la plupart des alliages Al-Si ne conviennent pas aux applications à haute température, car leur résistance à la traction et à la fatigue ne sont pas aussi élevées que souhaitées dans la plage de température 230-350 ° C, qui est la plage de températures souvent atteinte dans les composants du moteur automobile en service. Le principal défi réside dans le fait que la résistance des alliages d'aluminium moulé traitable thermiquement diminue à des températures supérieures à ~200 ° C. Les précipités de Mg2Si et Al2Cu qui maintiennent la résistance élevée de l'alliage habituellement grossissent ou se dissolvent à des températures supérieures à 150 ° C, ce qui entraîne une réduction des performances à haute température et par conséquent limite les applications pratiques. La plupart des alliages moulés Al-Si à ce jour sont destinés pour des applications à des températures qui ne dépassent pas environ 230 ° C. La résistance des alliages dans des conditions à haute température est améliorée grâce à l'obtention d'une microstructure contenant des intermétalliques thermiquement stables et résistants au grossissement, ce qui peut être obtenu avec l'addition de Ni. Zr et Sc. Le nickel conduit à la formation d'aluminiure de nickel Al3Ni et Al9FeNi en présence de fer. Tandis que le zirconium lui, forme des particules d’Al3Zr. Ces intermétalliques améliorent la résistance à la température élevée des alliages Al-Si. Certaines améliorations intéressantes ont été réalisées en modifiant la composition de l'alliage de base avec des additions de Mn. Ce qui entraîne une augmentation de la résistance et de la ductilité à la fois, à température ambiante et à haute température. Les principaux effets qui peuvent être obtenus en ajoutant du scandium aux alliages d'aluminium sont le raffinement du grain et le durcissement par précipitation à partir de particules Al3Sc. L'ajout de Zr avec le Sc ce traduit par une amélioration de la résistance d’alliage et la résistance au grossissement des précipités de durcissement dans la microstructure. Les alliages d'Al-Si-Cu-Mg tels que les alliages 354 (Al-9% Si-1,8% Cu-0,5% Mg, en poids) montrent une plus grande réponse au traitement thermique en raison de la présence de Mg et de Cu. Ces types d'alliage présentent d'excellentes valeurs en résistance aux températures basses et hautes. Les ajouts de Zr, Ni, Mn et Sc devraient maintenir la performance de ces alliages à des températures encore plus élevées. La présente étude a donc été réalisée pour étudier les effets des ajouts de Zr, Ni, Mn et Sc, individuellement ou en combinaison, sur la microstructure et les propriétés de traction de l'alliage de fonderie 354 à température ambiante et à haute température (250 °C) en utilisant différentes temps de maintien à la température de test. Nous avons préparé six alliages, en utilisant l'alliage 354 raffiné au Ti 0,2% en poids, comprenant l'alliage R (354 + 0,25% en poids de Zr) considéré comme l'alliage de base ou de référence et cinq autres, à savoir les alliages S, T, U, V et Z contenant diverses quantités de Ni, Mn, Sc et Zr, ajoutés individuellement ou en combinaison. À des fins de comparaison, un autre alliage (L) a été préparé à partir d'alliage 398 (Al-16% Si), rapporté pour donner d'excellentes propriétés à haute température, auxquelles ont été réalisés les mêmes niveaux d'ajouts de Zr et Sc, comme dans l'alliage Z. Des barreaux de traction, en utilisant un moule permanent ASTM B-108, ont été préparés à partir des différents alliages 354. Les barres d'essai ont été traitées par mise en solution, en utilisant un traitement thermique à une étape ou à plusieurs étapes, suivi de la trempe dans l’eau tiède, puis un vieillissement artificiel en utilisant différents traitements de vieillissement (T5, T6, T62 et T7). Le traitement de mise en solution à une étape (ou SHT 1), était établie à une durée de 5 h à 495 °C et le traitement de mise en solution à plusieurs étapes (ou SHT 2), comprenait 5 h à 495 ° C, suivi de + 2 h à 515 ° C et terminé par + 2 h à 530 ° C. Les essais de traction avec les barres d'essai brut de coulé et traités thermiquement ont été effectués à température ambiante à l'aide d'un taux de déformation de 4 x 10-4s-1. Cinq barres d'essai ont été utilisées par composition/condition thermique d'alliage. Dans ce cas, les barres d'essai ont été testées avec ou sans stabilisation préalable pendant 200 h à 250 °C. Les essais de traction à haute température ont été effectués à 250 °C, où les barres d'essai ont été stabilisées pendant 1 h et 200 h à 250 ° C avant l'essai. Les analyses thermiques des différents alliages 354 ont été réalisées pour déterminer la séquence de réactions et de phases formées lors de la solidification dans des conditions de refroidissement proches de l'équilibre. Les principales réactions observées comprenaient la formation du réseau dendritique α-Al à 598 °C, suivie de la précipitation de la phase eutectique Al-Si et la phase β-Al5FeSi post-eutectique à 560°C; la phase Mg2Si et la transformation de la phase βen phase π-Al8Mg3FeSi6 à 540 °C et 525 °C; et enfin, la précipitation de Al2Cu et Q-Al5Mg8Cu2Si6 presque simultanément à 498 °C et 488 °C. En raison du faible taux de solidification des pièces moulées durant l’analyse thermique et d'une teneur en Zr de 0,25% en poids, tous les alliages contenant du Zr sont situés dans la région L + Al3Zr du diagramme de phase Al-Zr pendant l'étape de fusion. Trois réactions principales sont détectées avec l'addition de Ni, c'est-à-dire la formation de phases AlFeNi, AlCuNi et AlSiNiZr. De plus grandes tailles de particules de phase AlFeNi et AlCuNi sont observées dans l'alliage T avec sa teneur en Ni supérieure de 4% en poids, par rapport à celles observées dans l'alliage S à 2% de teneur en Ni. L'ajout de Mn dans l'alliage U contribue à réduire les effets néfastes de la phase β-fer en le remplaçant par les phases intermétalliques du fer en forme script chinoise et de boues α-Al15(Fe,Mn)3Si2. Les phases intermétalliques du Sc observées dans cette étude apparaissent sous deux formes différentes: (Al, Ti) (Sc, Zr) et (Al, Si) (Sc, Zr, Ti). Avec l'utilisation du traitement en solution multi-étapes - impliquant des températures de solution plus élevées et des durées plus longues, une quantité accrue de fusion initiale devrait se produire. Le grossissement des particules de Si est également observée; les particules plus grandes augmentant aux dépens des plus petites. Des particules primaires de Si sont observées dans la microstructure de l'alliage hypereutectique L avec sa teneur élevée en Si de 16% en poids. Les données de traction ont montré que l'UTS et le pourcentage d'allongement des alliages R, S, T, U, V et Z augmentaient avec le traitement thermique de mise en solution à une étape par rapport à l'état brut de coulé. Le traitement thermique de mise en solution multi-étapes a montré des propriétés de traction plus élevées que celles obtenues avec le traitement SHT 1. L'utilisation du traitement T62, en incorporant le traitement en solution SHT 2, permet une dissolution maximale des phases de cuivre dans les multi-étapes du traitement de mise en solution, ce qui entraîne une amélioration maximale pour les UTS et YS. Sans stabilisation, les traitements T6 et T62 fournissent les meilleures améliorations pour les valeurs UTS et YS de tous les alliages. Les meilleures propriétés de traction des alliages testés à température ambiante après stabilisation à 250 °C pendant 200 h, sont obtenues avec le traitement thermique T6. Après le traitement T62, l’alliage U (contenant 0,75% en poids de Mn + 0,25% en poids de Zr) a montré l'augmentation maximale des valeurs UTS et YS. L'ajout d’élément d'alliages tel que Zr, Ni, Mn et Sc à un alliage 354 (Al-Si) améliore les propriétés de traction à haute température. L'alliage S (Al 354 + 0.25wt% Zr + 2wt% Ni) et l'alliage U (Al 354 + 0.25wt% Zr + 2wt% Ni) se comportent mieux en cas de conditions à haute température, avec une stabilisation d'une heure à 250 °C. Après 200 heures de stabilisation à 250 °C, la résistance des alliages traités avec T6 est considérablement réduite tandis que la ductilité augmente, l'alliage R représentant le pourcentage d'allongement le plus élevé, ~19%, suivi de l'alliage Z avec une ductilité de ~16%. La réduction de la résistance peut être attribuée à l'adoucissement de l'alliage, qui se produit après un aussi long temps de stabilisation à la température de test élevée. Bien que l'alliage L traité avec T6 affiche de bonnes valeurs de résistance, par rapport aux alliages 354 aux mêmes conditions de test à haute température, sa ductilité est de ~2,45% par rapport à l'alliage S qui produit une résistance à la traction similaire mais a une ductilité de ~6,5%. La réduction de la résistance peut être attribuée à l'adoucissement de l'alliage qui se produit après un si long temps de stabilisation à la température de test élevée.

碩士
國立成功大學
材料科學及工程學系
105
With Industrial development, The economic Crisis that oil Shortage and the Global warming cause is getting worse, In order to reduce the energy problem ,we can start from replacing the motorcycle engine block material. Normally the material of traditional engine block is gray cast iron, the problem of cast iron is its weight, the more weight of engine block is, and the more energy has to be consumed. So we decide to use light metal to replace the gray cast iron as the material of engine block. Motorcycle engine operated at 150 ℃~250℃ and piston rings move up and down reciprocally with the piston. The functions of a piston ring are to seal off the combustion pressure, to distribute and control the oil, to transfer heat, and to stabilize the piston. Therefore, the cylinder liner need excellence thermo-mechanical properties and wears properties. In this study, we utilize the spray-forming process combined with post process such as backward extrusion and high temperature heat treatment to produced high Si-containing aluminum alloys test sample(AC9A-30Si and AZFM-30Si), Base on previous statement. 250℃tensile test and wear test(170℃) was employed to study the Thermo-mechanical properties and establish the wear mechanism of high Si-containing aluminum alloys at elevated temperature . Beside the study of mechanical property and wear mechanism at elevated temperature, Also compare different post heat treatment parameter (Heat treatment Time) effect on mechanical and wear property . As a result we could established the optimum post process parameter for high Si-containing aluminum alloys engine block.

碩士
元智大學
機械工程學系
92
The purpose of this study was to determine the effect of the preaging treatment and predeformation treatment on the strength of Al—Mg—Si—Cu alloy. The 6013B alloy of higher Cu content occurred that the phenomenon of the grains over-grown in the conventional T6 heat treatment, and therefore the strength of the 6013B alloy was subjected to decrease, but both alloys similar hardness properties. Nevertheless, the phenomenon of feeble strength has been improved effectively by way of T8 treatment due to the deformation bends and subgrains were resulted from the 6013B alloy, the tensile strength can be achieved to 455MPa. The strength of the 6013A alloy is similar with the T6 treatment due to its deformed reduction insufficient lend to unable to produce the deformation bends and subgrains in T8 treatment. The high-strength 6013B alloy has been developed by way of T8P1 process, the tensile strength can achieve 499MPa, and than the tensile strength of the 6013A alloy can also be achieved to 478MPa. The R14T6 treatment of lower temperature grain refinement has been effectively improved the phenomenon of over-big grain and feeble strength in the 6013B alloy, and obtained a refined equiaxed grain that the grain size about 25μm, and than its tensile strength can also be achieved to 435MPa. Although the 6013A alloy can be refined the grain up to 27μm, but its mechanical properties has been not outstandingly increased than the conventional T6 treatment due to its grain refinement range isn’t large. In accordance with the principle of grain refinement, the both alloys have been obtained even more refined equiaxed grain by way of the R16T6 and R18T6 research. The microstructure was observed the phenomenon of immense crack and intercrystalline crack for as extrusion of the 6013A alloy after the transverse and longitudinal fishskin mode welding, and the liquation cracks were discovered that it is existed the low fusion point eutectic phase of Mg2Si with excess Si by way of high power optical microscope and scanning electron microscope and energy dispersion X-ray analysis. The R14T6 materials of the refined grain were also restrained that the liquation cracks were resulted after the grain refinement, the creases are merely decreased that the amount and length of the liquation cracks were arose.

Selective laser melting (SLM) is an additive manufacturing technology. In this thesis, a heat-treatable Al-3.5Cu-1.5Mg-1Si alloy and related materials (composites and hybrid materials) have been successfully fabricated by selective laser melting and characterized in terms of densification, microstructure, heat treatment, mechanical properties as well as tribological and corrosion behavior. Firstly, the fully dense SLM Al-Cu-Mg-Si alloy was fabricated by SLM successfully. The alloy shows a higher yield strength than SLM Al-12Si alloy, and lower wear resistance and corrosion rate than commercial 2024 alloy before and after T6 heat treatment. Secondly, with the aim of designing new alloy compositions and to examine the phases and microstructures of SLM Al-Cu alloys and to correlate their microstructures with the observed mechanical properties, Al-xCu (x = 4.5, 6, 20, 33 and 40 wt. %) alloys have been synthesized in-situ by SLM from mixtures of Al-4.5Cu and Cu powders. The results indicate that the insufficient Cu solute diffusion during the layer-by-layer processing results in an inhomogeneous microstructure around the introduced Cu powders. With increasing Cu content, the Al2Cu phase in the alloys increases improving the strength of the material. These results show that powder mixtures can be used for the synthesis of SLM composites but the reaction between the matrix and the second-phase should be considered carefully. Thirdly, the TiB2/Al-Cu-Mg-Si composite was also designed and fabricated successfully by SLM and it shows a higher strength than the unreinforced SLM alloy before and after T6 heat treatment. Finally, an Al-12Si/Al-3.5Cu-1.5Mg-1Si hybrid with a good interface was fabricated successfully. This hybrid alloy shows a good yield strength and elongation at room temperature, indicating an effective potential of selective laser melting in the field of hybrid manufacturing.

碩士
國立成功大學
材料科學及工程學系碩博士班
100
Communication connectors are almost manufactured by leaded brass. However, using lead easily causes environmental pollution, so developing substituted material is the big issue. Aluminum alloys have many advantages, such as light, low-cost, high specific strength, and environmental-friendly. Thus aluminum alloys are preferred for environmental-friendly material. The continuous chips easily disturb the cutting process. Hence chip segmentation is the important indicator for machinability. 　　4384 Al-Si-Cu-Ni-Mg aluminum alloy has excellent chip breakability in aluminum alloys as a result of many hard and brittle second-phases. In order to achieve the chip breaking standard of leaded brass, we use aging treatment to increase the chip breakability of 4384 Al-Si-Cu-Ni-Mg alloy. One part of this study probes effects of aging treatment to chip formation on 4384 Al-Si-Cu-Ni-Mg aluminum alloy. It shows that the artificial peak-aging treatment can supply the best chip breakability. For satisfying application, the other part of this study discusses that the feasibility of 4384 Al-Si-Cu-Ni-Mg aluminum alloy can apply in the communication connectors by comparing material characteristics of 4384 Al-Si-Cu-Ni-Mg aluminum alloy and C3604 leaded brass. 　　The α-Al hardness value of 4384 Al-Si-Cu-Ni-Mg aluminum alloy is directly proportional to chip breakability. When the deformation resistance of α-Al increase, working hardening layer decrease. In cutting process, α-Al is easy to break, which could reduce the cutting chip size, so the artificial peak-aging treated 4384 aluminum alloy has the best chip breaking factor in this material. 　　The material characteristics of 4384 Al-Si-Cu-Ni-Mg aluminum alloy enhance by artificial peak-aging treatment. The hardness, yield stress and chip breakability are more close to C3604 leaded brass. Weather resistance and electromagnetic shielding properties are even beyond C3604 leaded brass. Therefore, the artificial peak-aging treated 4384 Al-Si-Cu-Ni-Mg aluminum alloy is expected to replace the leaded brass and applied in communication connectors.

The microstructural evolution and mechanical properties at elevated temperatures of a recently fabricated fine-grained AA6xxx aluminium sheet were evaluated and compared to the commercially fabricated sheet of the same alloy in the T4P condition. The behaviour of the fine-grained and T4P sheets was compared at elevated temperatures between 350°C and 550°C, as well as room temperature. Static exposure to elevated temperatures revealed that the precipitate structure of the fine-grained material did not change extensively. The T4P material, however, underwent extensive growth of precipitates, including a large amount of grain boundary precipitation. At room temperature, the T4P material deformed at much higher stresses than the FG material, but achieved lower elongations. Deformation at elevated temperatures revealed that the fine-grained material achieved significantly larger elongations to failure than the T4P material in the temperature range of 350°C-450°C. Both materials behaved similarly at 500°C and 550°C. Above 500°C, the grain size was greatly reduced in the T4P material, and only a slightly increased in the fine-grained material. At temperatures above 450°C, the elongation to failure in both materials generally increased with increasing strain-rate. The poor performance of the T4P material at low temperatures was attributed to the precipitate characteristics of the sheet, which lead to elevated stresses and increased cavitation. The deformation mechanism of both materials was found to be controlled by dislocation climb, accommodated by the self diffusion of aluminium at 500°C and 550°C. The deformation mechanism in the fine-grained material transitioned to power law breakdown at lower temperatures. At 350°C to 450°C, the T4P material behaved similarly to a particle hardened material with an internal stress created by the precipitates. The reduction in grain size of the T4P material after deformation at 500°C and 550°C was suggested to be caused by dynamic recovery/recrystallization. The role of a finer grain-size in the deformation behaviour at elevated temperatures was mainly related to enhanced diffusion through grain boundaries. The differences in the behaviour of the two materials were mainly attributed to the difference in the precipitation characteristics of the materials.

博士
國立中央大學
機械工程學系
101
In recent years, aluminum alloys are widely used for making 3C products due to their lightweight, good mechanical properties and superior cosmetic appearance after anodizing. The purpose of this study is to introduce the anodization behavior of high purity aluminum and the qualities of anodized Al-Mg-Si and Al-Zn-Mg-Cu aluminum alloy with different matrix structures. The individual brief are as following: Gas bubbles formed during anodization of aluminum in sulfuric acid solution but no precise computation of bubbles size have been presented and discussed in literatures. In this study, we utilized X-ray photoelectron spectroscopy (XPS) to investigate the constituent phases of anodic alumina oxide that formed in a short anodization time. Amorphous alumina prevailed in the films together with a certain amount of hydrated alumina (Al(OH)3), oxyhydroxide (AlOOH) and alumina sulfate (Al2(SO4)3). A hydrophone was applied to measure the acoustic pressure field in the electrolyte during anodization. Experimental results indicated that acoustic pressure oscillated with appropriate frequency ranging from 50 Hz up to 500 Hz. A power spectrum density function (PSD) and cepstrum was adopted to analyze the signal of sound spectrum. The collapse and rebirth of gas bubbles on the surface of anodic aluminum oxide (AAO) film and/or at the pore bases oscillated the measured acoustic pressure. We found that the size of gas bubbles could range from 8 nm to 16 nm in diameter to corresponding to the detected oscillated frequency 400 to 50 Hz, respectively. A series of reactions occurred during anodization to yield SO3 gas and O2 gas, which collapsed and rebirth during the early-growth of AAO film. This study also intends to investigate the effect of deformation on varying the matrix of 6063 and 7075 alloy and on the quality of anodized aluminum oxide film.The Al-Mg-Si alloy and Al-Zn-Mg-Cu contains about 0.6 %Mg and 0.4%Si and Al- 5.4 wt% Zn- 2.5 wt% Mg- 1.5 wt% Cu are a heat treatable alloy with precipitation hardening. Different degree of deformation would be introduced to simulate the forming process in making parts resulting in varying effects on the precipitation hardening of matrix. The highest dislocation density was observed in the SCRT6 sample which also showed the lowest inclusion particle numbers among the three Al-Mg-Si samples. The sample that has been subject to the cryo-rolling process prior to rolling and heat treatment showed few subgrains and smaller amounts of second phase particles in the matrix than was the case with the other two sets of Al-Zn-Mg-Cu samples. Subsequently, all samples were anodized in a 15 wt % sulfuric acid solution for different time spans to obtain different state of AAO films. The anodized samples were further analyzed and observed with Field emission scanning electron microscope (FE-SEM), Transmission electron microscopy (TEM), Atomic force microscopy (AFM) and Electrochemical impedance spectroscopy (EIS) and potential-dynamic polarization curve (PD-curve) analysis. We determined that the constituent phases in the AAO film were composed of hydrated amorphous alumina, hydrated oxide (Al(OH)3) and oxyhydroxide (AlOOH) phases together with some silicon-containing particles trapped in the films on Al-Mg-Si samples. The matrix of Al-Zn-Mg-Cu alloy contained the disc-like precipitates of Mg2Zn and Al2Cu and dissolved from the matrix during anodizatione and remained tiny air-pockets in the AAO film. In the potential-dynamic polarization test, the silicon-containing particles and tiny air-pockets that existed in the AAO films were found to inversely influence the corrosion resistance of the anodized samples.